Horizontal axis wind turbines usually include a tower carrying on its top a nacelle, and a rotor mounted on a drivetrain which is housed in the nacelle, The drivetrain usually includes a main shaft, a gearbox and a generator. In many turbines, the main shaft is carried, via two axially offset bearings, by a main bearing housing. The connection between the main shaft and the gearbox input shaft can be of any suitable type, for example it could be a rigid connection, or it could include a low speed coupling as exemplified in WO2012052022A1 allowing some flexibility in the alignment of the shafts. In some cases, the main bearing housing, the gearbox and the generator each have their respective direct support to a carrying structure of the nacelle. In other cases the gearbox and the generator are cantilevered, as exemplified below, from the main bearing housing, and in such cases the joint between the main bearing housing and a fixed body of the gearbox will experience relatively large forces. In addition, certain types of low speed couplings have a relatively large diameter. This will require a large diameter of any fixed connection between the main bearing housing and the fixed body of the gearbox. For example, the main bearing housing and the gearbox might be connected via relatively large cover for a low speed coupling between the main shaft and the gearbox input shaft. However, it might be desired to provide a main shaft that has a relatively small diameter at its connection to the gearbox, and therefore a small bearing between the main shaft and the main bearing housing.